Support non browse-kill-ring overlays in the *Kill Ring* buffer.

This should fix issue https://github.com/browse-kill-ring/browse-kill-ring/issues/46 which is caused by `global-linum-mode` adding additional overlays into the `*Kill Ring*` buffer.  There were a few places in the code where we called `overlays-at` and assumed that the only overlays we would ever get back would be from `browse-kill-ring`.  This patch series removes all of these assumptions, one function at a time, and instead uses the already existing `browse-kill-ring-target-overlay-at` function, which finds the correct overlay.